Automatic TIG Welding Process for Titanium Tube-to-Tubesheet Joints

Literature Overview

This paper, published in Welding (Issue 9, 2007, pp. 48-50), authored by Zou Liwei from the Harbin Welding Research Institute, Gao Lei and Zhang Yingying from Liaoning Petrochemical University, and Su Wei from Liaohe Petroleum Exploration Bureau Oilfield Engineering Construction Company No. 2, presents a practical case study on the automatic TIG welding of titanium tube-to-tubesheet joints for a gas cooler on an offshore platform. Both the heat exchange tubes and tubesheet are fabricated from commercially pure titanium TA2, and the study details the process development that resolved critical contamination and embrittlement issues in the weld zone.

Core Technical Content

The research addresses a real-world engineering challenge in offshore oil and gas production equipment, where titanium heat exchangers are specified for their excellent corrosion resistance in chloride-containing marine environments. The tube-to-tubesheet joint is a critical structural and functional interface that must withstand both mechanical loads and corrosive attack over the equipment's service life.

Joint Configuration and Welding Approach

The joint configuration involves a fillet weld with filler wire (TIG weld with filler rod) connecting the titanium tube to the tubesheet. This is a butt-fillet joint where the tube is inserted into a drilled hole in the tubesheet, and the weld is applied to the outer diameter to create a hermetic seal and mechanical bond.

Design Parameter	Specification
Base material	TA2 commercially pure titanium
Tube specification	Typically 25-50 mm OD, 1.5-3.0 mm wall
Tubesheet thickness	30-80 mm
Joint type	Tube-to-tubesheet fillet weld
Welding process	Automatic TIG with filler wire
Shielding	Dual gas protection (front and back)
Filler wire	ER-Ti-2 (matching TA2 composition)

Critical Process Parameters

The study identifies several critical process parameters that must be carefully controlled to achieve a sound weld:

Parameter	Optimized Value	Criticality
Welding current	120-180 A	High - controls penetration and bead size
Welding speed	20-40 mm/min	High - controls heat input and bead geometry
Main nozzle gas flow	15-20 L/min	Critical - prevents front-side oxidation
Back purge gas flow	10-15 L/min	Critical - prevents backside oxidation
Filler wire diameter	1.6-2.4 mm	Medium - affects deposition rate
Electrode diameter	3.2 mm	Medium - affects arc stability
Arc length	2-3 mm	High - affects bead quality and gas coverage
Travel angle	90° (perpendicular)	Medium - affects bead symmetry

Contamination Control and Embrittlement Prevention

The primary challenge in titanium tube-to-tubesheet welding is the prevention of atmospheric contamination. Titanium becomes highly reactive with nitrogen, oxygen, and hydrogen at temperatures above approximately 400°C. Contamination leads to:

Oxidation: Formation of brittle TiO₂ and TiN compounds in the weld and HAZ, causing loss of ductility and potential cracking.
Nitridation: TiN formation creates hard, brittle phases that reduce toughness and can initiate cracking under stress.
Hydrogen pickup: Dissolved hydrogen causes delayed cracking and reduced ductility.
Color indicators: Blue, gray, or dark purple discoloration on the weld surface indicates oxidation; the weld should be bright silver or straw-colored for acceptable quality.

The study's solution involved implementing a comprehensive gas protection system:

Front-side shielding: High-purity argon (99.99%) delivered through the TIG torch nozzle at 15-20 L/min to protect the weld pool and hot metal from atmospheric contamination.
Back-side purge: Argon purge applied to the interior of the tube or the backside of the tubesheet to protect the root side of the weld.
Pre-weld cleaning: All titanium surfaces must be cleaned with acetone or a titanium-compatible cleaning agent, and all organic contaminants must be removed before welding.
Post-weld cooling: Controlled cooling rate to prevent hydrogen pickup during the post-weld cooling phase.

Engineering Practice Integration

In offshore platform applications, titanium heat exchangers are subject to demanding service conditions including high-pressure operation, elevated temperatures, and aggressive chloride-containing process fluids. The tube-to-tubesheet joint must maintain integrity under cyclic thermal loading and pressure fluctuations. The automatic TIG welding process described in the paper offers several advantages for this application:

Consistency: Automated processes produce uniform weld beads with minimal operator variability, which is critical for maintaining consistent mechanical properties and corrosion resistance.
Repeatability: Once qualified, the process can be repeated with high confidence, reducing the need for extensive non-destructive testing.
Productivity: Automated welding significantly increases throughput compared to manual welding, which is important for large-scale heat exchanger fabrication.
Quality traceability: Automated systems can log all process parameters in real-time, providing a complete quality record for each weld.

Quality Assurance and Inspection

For titanium tube-to-tubesheet joints in offshore applications, a comprehensive quality assurance program is essential:

Inspection Method	Purpose	Acceptance Criteria
Visual inspection	Surface quality, discoloration	No blue/gray discoloration, no cracks
Dye penetrant testing (PT)	Surface-breaking defects	No indications above 0.5 mm
Radiographic testing (RT)	Internal porosity, incomplete fusion	No porosity > 0.5 mm, no incomplete fusion
Ultrasonic testing (UT)	Joint integrity, bond quality	No indications of lack of bond
Hydrostatic test	Leak tightness	No leakage at 1.5x design pressure

Study Insights and Implications

This case study provides valuable practical guidance for titanium tube-to-tubesheet welding in offshore applications. The key insight is that successful welding of titanium requires not just proper welding parameters but also a comprehensive approach to contamination control, including thorough surface preparation, adequate gas protection on all exposed surfaces, and controlled post-weld cooling. For engineering practice, the automatic TIG welding process described here should be considered the baseline approach for titanium heat exchanger fabrication, with manual welding reserved for repair applications where automation is not feasible. The emphasis on dual gas protection (front and back) is particularly important and should be considered mandatory for all production welding of titanium components.